1. Field of the Invention
The present invention relates to a line pressure control apparatus for a V-belt type continuously variable transmission, which regulates the upper limit of line pressure during high-speed engine revolution.
2. Description of the Prior Art
Conventionally, a V-belt type continuously variable transmission (hereinafter referred to as “belt CVT”) provided in a vehicle is constructed such that a primary pulley and a secondary pulley whose groove widths are variably controlled according to oil pressure support a V belt by sandwiching it, and power is transmitted by contact frictional force of the V belt.
An example of the belt CVT has been disclosed in Japanese Laid-Open Patent Publication No. 11-37237. In the belt CVT disclosed in this publication, the thrust of pulleys is found based on the input torque and the gear ratio, and it is converted to oil pressure value according to predetermined values indicative of the pressure receiving areas of the secondary pulley and the primary pulley, or the like, and the oil pressure value is supplied as target line pressure to a transmission mechanism.
As the transmission mechanism, a mechanical feedback mechanism and a gear ratio control mechanism have been disclosed which cause a step motor to open a shift control valve according to the target gear ratio, and cause a servo link, to which the primary pulley is connected, to close the shift control valve according to the groove width of the primary pulley to finish changing gears when the groove width of the primary pulley reaches a value corresponding to the target gear ratio.
Further, in the belt CVT a line pressure is generated by regulating an oil pressure supplied from an oil pump. The line pressure is supplied to the secondary pulley, and a primary pressure obtained by regulating the line pressure is supplied to the primary pulley via the shift control valve.
The above described conventional belt CVT, however, has the problem that cavitation noise occurs in the oil pump, which generates line pressure for use in the continuously variable transmission, during high-speed engine revolution.